Realizing electronically adjustable inductors or capacitors in large-signal applications enables the design of tunable oscillators and filters in various applications, which have no alternatives until now. One is interested in a high efficiency, a stable reliable operation and in low implementation cost in such applications. The design of switched circuit concepts, that fulfill all these requirements and remain operational far into the high frequency range, is a real challenge. First, the basic physical network conditions shall be met. In addition, it is important to consider component properties, which influence the network characteristics significantly. Controlled reactances based on switched capacitors or switched inductors have low losses and are used in CRT horizontal deflection. U.S. Pat. No. 4,533,855 shows how to control the resulting total capacitance of two coupled capacitors by varying the coupling interval during a period of a resonant circuit electronically. FIG. 1 shows the main circuit wherein the capacitor CS and the inductance LH form a series resonant circuit. The capacitor CM is coupled by a controlled switch transistor Q1 and a diode D1 to capacitor CS in a part interval of the resonant circuit period. The coupling control is operative during one half period of the resonant circuit, because during the other half period D1 is always conductive. The capacitor CR is not relevant because it is short-circuited by the transistor QH and diode DH. The transformer DR develops at its secondary winding a voltage pulse during the retrace interval, which depends only on the control signal from QH. This results in a pulse-width modulation signal (in the following named as PWM signal) at the control input of Q1, which is directly coupled to the control signal QH (h drive). When the PWM signal at the input (mod) is changed, the PWM interval length varies but not its phase angle with respect to h-drive.
U.S. Pat. No. 6,586,895 shows how to control an inductor or a higher order network using a variable coupling interval during both half waves of a resonant circuit period. FIG. 2 shows the main circuit wherein the capacitor CS and the inductance LH form a series resonant circuit. The capacitor CM and the inductor LM are both coupled via the controlled transistors Q1a and Q1b and their integrated body diodes to CS in two part-intervals of the resonance circuit period. The coupling control operates in both half cycles of the resonant circuit period, since the current in CM and LM depends in both directions from the control of the transistors Q1a respectively Q1b. The capacitor CR is not relevant, because it remains short-circuited by the transistor QH and diode DH, or the transistors Q1a and Q1b remain fully open or fully closed when QH is open.
In the approaches mentioned above a PWM signal is generated, which is synchronized with the deflection frequency. This is done by means of a sawtooth generator, which is directly connected to the input signal h-drive or alternatively synchronized over the horizontal retrace pulse. In all of these solutions the PWM signal remains unaffected from the controlled capacitance or inductance and the resulting resonance circuit period. The PWM modulator is therefore only controlled by the deflection frequency (h-drive) and the input (mod). If one wants to tune the resonance frequency in an LC resonant circuit by means of a controlled coupling interval of capacities or inductances characterized by resulting substantially equal half-periods, the coupling interval must remain in phase with the resonance circuit period, or at least remain in phase with the corresponding half-period. A coupling control signal, which is independent from the resonant circuit period can no longer be used to generate the switching signals for the transistors.
The generation of control signals, which shall be dependent on the controlled signal itself is very difficult. The main problem is that altering the output signal instantly changes the input value and can thus make the system unstable. In this case, the frequency change in the resonant circuit has a direct influence on the coupling control. An integration or low-pass filtering of the control variables for the sawtooth generator can minimize this positive feedback behavior and stabilize such systems. The eminent disadvantage of this method is a poorer dynamic behavior with respect to the control input. The system transient response becomes slower with respect to the control input. The capacitance and inductance can be varied only as fast as the control signals can be updated. It is desired that frequency tuning of tunable resonant circuits or filters is only dependent on one single input control variable. This means that amplitude variations in current or voltage shall not affect the frequency. In other applications the amplitude in a resonant circuit shall be controlled or regulated independently from the resonant circuit frequency.
Therefore, in a tunable resonant circuit using controlled interval coupling for its component variation, it is therefore very important that the PWM signal can be controlled independently from the current- or voltage amplitude in the resonant circuit. This means the resulting resonance frequency is a function of a control variable only. Capacitance or inductance using controlled interval coupling in resonant circuits or generally many other inductor and capacitor circuits often generate a current flow through diodes, which represent often the integrated body diodes of switches. The conduction loss of the diodes is proportional to the diode threshold voltage and current. Since these resonant circuits are often one of a zero voltage switching (ZVS) or zero current switching (ZCS) concept, the losses are mainly determined by the switch and the diode conduction losses. Furthermore, the diodes generate transients in the transition interval from blocking to conduction mode and vice versa, interferences and therefore additional losses. Therefore, in efficient circuit design concepts it is therefore important that all these factors can be avoided or at least be minimized. The following invention describes a method and their detailed implementations to control capacities or inductances electronically. It fulfills all the requirements above and is characterized by the fastest possible transient response regarding the control behavior. A capacitance or inductance can be varied in its entire dynamic range from one resonance circuit half period to the other one.